The present invention relates generally to computer processing systems, and more specifically, to instruction filtering.
An instruction pipeline in a computer processor improves instruction execution throughput by processing instructions using a number of pipeline stages, where multiple stages can act on different instructions of an instruction stream in parallel. A conditional branch instruction in an instruction stream may result in a pipeline stall if the processor waits until the conditional branch instruction is resolved in an execution stage in the pipeline before fetching a next instruction in an instruction fetching stage for the pipeline. A branch predictor may attempt to guess whether a conditional branch will be taken or not. Branch target prediction attempts to guess, based on prior execution result, a target of a taken conditional or unconditional branch before it is computed by decoding and executing the instruction itself. A branch target may be a computed address based on an offset and/or an indirect reference through a register.
A branch target buffer (BTB) is used to predict the target of a predicted taken branch instruction based on the address of the branch instruction. Predicting the target of the branch instruction can prevent pipeline stalls by not waiting for the branch instruction to reach the execution stage of the pipeline to compute the branch target address. By performing branch target prediction, the branch's target instruction decode may be performed in the same cycle or the cycle after the branch instruction instead of having multiple bubble/empty cycles between the branch instruction and the target of the predicted taken branch instruction. Other branch prediction components that may be included in the BTB or implemented separately include, but are not limited to, a branch history table and a pattern history table for predicting the direction of the branch. A branch history table can predict the direction of a branch (taken vs. not taken) as a function of the branch address. A pattern history table can assist with direction prediction by associating a direction prediction for the given branch with the path of branches that was encountered in reaching the given branch that is to be predicted.
Instructions supporting branch prediction are examples of non-functional accelerator type instructions that are not critical to correct program flow but can enhance program flow. Repeated execution of certain non-functional accelerator type instructions can impact overall processing throughput and the effectiveness of accelerator functions.